What a 1930s observatory sees that satellites miss

The dome creaks when it turns. The telescope inside was built before anyone had flown in a jet, and its mirror is smaller than a dinner table. Yet every clear night, a small team still opens the shutter and points it at the same patches of sky they have watched since the 1930s.

It sounds like nostalgia. It is actually one of the most useful things astronomy has: a long, steady record of how certain stars change over decades.

Why an old record matters

Many stars brighten and dim over years or even centuries. A satellite that operates for a decade can catch part of that story. A glass plate archive that goes back ninety years can show the whole shape of it.

Satellites take beautiful pictures. We take the same boring picture every clear night for ninety years.

The observatory’s director, Mireille Kante, says that line with a smile, but she means it. The value is in the repetition. When a star suddenly behaves strangely, researchers around the world write to ask what it did in 1954.

How the old plates are being saved

Until the 1990s every observation was recorded on glass photographic plates. There are more than 60,000 of them in the basement, each in a paper sleeve with handwritten notes. Volunteers are scanning them one at a time on a specially built flatbed scanner.

• Each plate takes about four minutes to clean, scan and label.

• Handwritten logbooks are typed up so the time and weather of every exposure are searchable.

• Cracked plates are scanned in pieces and stitched back together in software.

• The finished scans are free for anyone to download.

What satellites still do better

None of this makes the old telescope better than a space mission. Satellites see without air in the way, work in daylight and can measure light that never reaches the ground. The two kinds of instrument answer different questions.

The small hilltop dome answers one question very well: what has this part of the sky been doing for most of a century? For some stars, that is the only question that matters.

Why it still needs people

Much of the work could be automated, and some of it is. The telescope now points itself and a camera has replaced the glass plates. But the team still insist on a person in the dome on every observing night.

The reason is weather. Thin cloud, dew on the mirror or a sudden gust can ruin a night’s data without any sensor noticing. A human who has spent years on the hill knows when to close the shutter and when to wait ten minutes.

The observatory runs public evenings once a month from October to March. Tickets are free and go quickly. If the sky is clear you will look through an eyepiece older than your grandparents; if it is not, you will hear stories about the nights when everything almost went wrong.